1. Field of the Invention
This invention relates generally to a circuit for measuring the rise and fall time of a digital electrical signal without using an oscilloscope.
2. Description of the Related Art
Many computers and other devices on the market today include digital logic circuitry. This circuitry allows the computer or device to perform functions specified by the user. Digital logic circuitry operates using only two different voltage signals, representing a logic xe2x80x9czeroxe2x80x9d and xe2x80x9cone.xe2x80x9d As such, digital logic circuits are often switching between an xe2x80x9conxe2x80x9d state (e.g. a logic xe2x80x9c1xe2x80x9d) and an xe2x80x9coffxe2x80x9d state (e.g. a logic xe2x80x9c0xe2x80x9d). The rate at which the logic circuitry can perform functions is partially dependent on the rate at which the circuitry can switch between logic states.
Digital logic circuits do not switch between a xe2x80x9c0xe2x80x9d to a xe2x80x9c1xe2x80x9d state immediately. There is an inherent delay associated with the switching process which includes rise and fall time. Rise time is the time it tales a digital logic circuit to transition from low to high logic level (e.g. xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d), and fall time is the time it takes to transition from a high to a low logic level (e.g. xe2x80x9c1xe2x80x9d to a xe2x80x9c0xe2x80x9d). Each different digital logic circuit has a different associated rise and fall time. It is important for the fabricator of a device including digital logic to be able to measure these rise and fall times, so that he may know exactly how fast the device will operate.
Traditionally, when one wanted to determine the characteristics of a particular circuit, the circuit would be connected to an oscilloscope and its waveform shape would be observed. By this observation, the approximate rise and fall time of the circuit can be determined. As noted above, this information is useful in calculating the speed associated with a particular circuit.
However, oscilloscopes are costly devices which often include complex circuitry. Hence, there is currently a need for a small simple device which can measure the rise and fall times of a digital signal produced by an electrical circuit.
The present invention is designed to provide a simple testing circuit capable of determining the rise and/or fall time of an output signal of a digital logic circuit. The digital logic circuit is supplied with a test signal from the testing circuit. The output signal from the digital logic circuit is compared with two threshold values for each of a rising and/or falling edge of the output signal. Based on when the output signal crosses the threshold values, a first set of waveforms are created for each of the rising and/or falling edge of the output signal which are then input to a digital logic array. The logic array produces a second set of waveforms indicative of the rise and/or fall time of the digital logic circuit. The second set of waveforms are then respectively integrated to determine their average value. This average value is then used in a calculation to determine the rise and/or fall time of the digital logic circuit. The testing circuit which accomplishes this measurement is simple and inexpensive. Hence, by using the testing circuit of the present invention, one can measure the rise and fall time of digital logic circuit without the need for expensive equipment, such as an oscilloscope.
The components of an electrical circuit of the invention for measuring rise and fall time have their own associated rise and fall time, which may affect the accuracy of the device. However, such problems are avoided in the invention because the same measurement circuitry is used for all measurements. Therefore, measurement circuit errors are common to all waveforms, and are mathematically canceled out in subsequent calculations.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.